The present invention relates to novel aromatic polyesters capable of being subjected to melt-processing, having high strength and high modulus properties and exhibiting reduced anisotropy when formed into films or sheets.
Recently there has been increasing demand for materials which exhibit high modulus and superior resistance to heat and to chemicals when formed into any of fibers, films and shaped articles. Although conventional polyesters are in use for various shaped articles, many of them are inferior in mechanical properties such as flexural modulus and so they have heretofore been unsuitable for uses requiring such high strength and high modulus properties. As a method for improving such mechanical properties, it has been known to incorporate fillers or reinforcing materials, e.g. calcium carbonate or glass fibers, into conventional polyesters. However, such conventional method causes problems in practical use; for example, the blended fillers or reinforcing materials would spoil the light weight characteristic, which is a feature of plastics, because of increasing specific gravity of the resulting blend, and molding machines would be subjected to heavy wear during molding operations.
Liquid crystal polyesters have come to be noted recently as melt-processable polymers suitable for uses requiring high strength and high modulus properties without fillers or reinforcing materials. Special attention has been paid to such liquid crystal polyesters since W. J. Jackson made public thermoplastic liquid crystal polyesters comprising polyethylene terephthalate and hydroxybenzoic acid in U.S. Pat. No 3,804,805 and Journal of Polymer Science Polymer Chemistry Edition, Vol.14, page 2043 (1976). Since then, various liquid crystal polyesters have been developed and studied with a view to attaining both improvement of mechanical properties, and melt-processability. However, these liquid crystal polymers have been unsuitable for such uses as films and sheets because their films or sheets exhibit a marked anisotropy in their mechanical properties due to orientation of the polymer molecules parallel to the flow direction in the melt.
As means for eliminating such anisotropy, the use of a cholesteric liquid crystal polymer has been proposed by W. R. Krigbaum et al (see U.S. Pat. No. 4,412,059). In this patent, however, although liquid crystal polyesters are noted, no description is found as to whether the said polyesters exhibit cholesteric liquid crystalline mesophase or not. Further, as will be described later, in order to obtain a film or sheet having high strength and high modulus properties, and reduced anisotropy, several specific characteristics must be required for the cholesteric liquid crystal polymers. But it is quite unknown whether the polyesters mentioned by Krigbaum have such characteristics or not. Moreover, there are not disclosed optically active diols as an optically active component in the above U.S. patent.
For obtaining a high strength and high modulus film or sheet with reduced anisotropy by melt-processing, a cholesteric liquid crystal polymer must satisfy several important requirements, which are as follows. (1) The said polymer should have high strength and high modulus properties in fibers when it is processed in a nematic state. (2) The cholesteric liquid crystal should take a laminar structure in which polymer molecules lie parallel to the film or sheet surface (the cholesteric helical axes are perpendicular to the surface). (3) Polydomain texture composed of a great number of small birefringent regions similar to polycrystalline structure must be transformed into a Granjean planar texture.
The above requirements are not referred to at all in the foregoing U.S. Pat. No. 4,412,059 in connection with the thermotropic liquid crystal polyesters disclosed therein. Therefore, it has been keenly desired to develop a cholesteric liquid crystal polyester capable of satisfying the aforementioned require ments to obtain a high strength and high modulus film or sheet with reduced anisotropy.